1. Field
Apparatuses and methods consistent with exemplary embodiments relate to a radio-frequency (RF) coil and a magnetic resonance imaging (MRI) apparatus employing the same, and more particularly, to a phased array RF coil having an improved structure for improving uniformity of an RF magnetic field and an MRI apparatus employing the same.
2. Description of the Related Art
A magnetic resonance imaging (MRI) apparatus obtains an image of a cross section of a human body using nuclear magnetic resonance (NMR). The atomic nuclei present in a human body, e.g., hydrogen (1H), phosphorus (31P), natrium (23Na), and carbon isotropes (13C), have unique rotating magnetic field constants according to NMR. Thus, an image of the inside of a human body may be obtained by applying electromagnetic waves toward the human body to cause magnetization vectors of the atomic nuclei to resonate and receiving a magnetic resonance signal generated by the magnetic vectors lying in a vertical plane due to the resonance. In this case, a radio-frequency (RF) coil is used to apply electromagnetic waves toward the human body to cause the magnetization vectors of the atomic nuclei in the human body to resonate and receive the magnetic resonance signal generated by the magnetic vectors lying in the vertical plane due to the resonance. One RF coil may be used to cause the magnetization vectors to resonate (transmission mode) and to receive the magnetic resonance signal (reception mode). Otherwise, two coils, i.e., an RF coil only for the transmission mode and an RF coil only for the reception mode, may be used to perform the transmission mode and the reception mode. A coil that may be used to perform both the transmission mode and the reception mode is referred to as a transceiving coil, a coil used to perform only the transmission mode is referred to as transmitting coil, and a coil used to perform only the reception mode is referred to as a receiving coil. In general, since the transmitting coil is installed inside a housing of the MRI apparatus, the transmitting coil is formed on a cylindrical (or oval-shaped) frame having a size such that a human body may be placed therein. On the other hand, since the receiving coil is generally attached onto a human body, the receiving coil may be thus formed according to a shape of a desired part of the human body, e.g., a head, neck, or waist.
A signal-to-noise ratio and luminous uniformity of a magnetic resonance image are important factors in evaluating the quality of the magnetic resonance image. The signal-to-noise ratio of the magnetic resonance image is proportional to a main magnetic field, i.e., a magnetic field of an element of the MRI apparatus such as a superconductive electromagnet or a permanent magnet. However, when the main magnetic field increases in a human body, electromagnetic waves decrease in terms of intensity and are delayed, thereby preventing an RF magnetic field from being uniformly formed. When the RF magnetic field is not uniformly formed in the human body, the degree of uniformity of the magnetic resonance image may be lowered. In particular, if the uniformity of a magnetic field in the transmission mode is low, both the degree of uniformity of the magnetic resonance image and the contrast of the magnetic resonance image are degraded.
In an MRI apparatus using a high magnetic field of about 3.0 tesla, a phased array RF coil in which a plurality of RF coils are arranged may be used to form a uniform RF magnetic field in a human body. The uniformity of an RF magnetic field may be increased in a human body by independently controlling the intensities and phases of RF signals supplied to coil elements included in the phased array RF coil. This technique is referred to as B1 shimming.
In general, a phased array transceiving coil or a phased array transmitting coil is formed on a cylindrical or oval-shaped frame that accommodates a human body. Representative examples of a cylindrical RF transceiving/transmitting coil are a birdcage RF coil and a transverse electromagnetic (TEM) coil. Although these coils are each originally designed as a single coil other than a phased array coil, they may be changed into phased array type coils through structural modifications.